Crystal growing apparatus

ABSTRACT

APPARATUS FOR USE IN GROWING LARGE SIZE MONOCRYSTALLINE PLATES OF MATERIALS SUCH AS ALUMINA. THE APPARATUS CONSISTS OF A FORMING MEMBER FOR WHICH THE PLATE IS GROWN AND HEAT SUSCEPTOR MEANS SHAPED SO AS TO PROVIDE A TEMPERATURE PATTERN SYMMETRICAL WITH THE EDGE CONFIGURATION OF THE PLATE.

'H. E; LABELLE, .13., ETAL Oct. 31, 1972 CRYS TAL GR OWI NG APPARATUS Filed Sept. 23. 1970 2 Sheets-Sheet 1 PU LLI N G MECHANISM 0 U a b 0 E d m H ATTORNEYS.

Oct. 31, 1972 H. E.- LABELLE, JR, ETAL 3,701,636

CRYSTAL GROWING APPARATUS Filed Sept; 25, 1970 E w gig m7 4 QNK m i l Ham/0 E Label/e, Jr. Wf/liam T. L/ff/e NINE/V7093 United States Patent 3,701,636 CRYSTAL GROWING APPARATUS Harold E. La Belle, Jr., Quincy, and William T. Little, Millis, Mass., assignors to Tyco Laboratories, Inc. Filed Sept. 23, 1970, Ser. No. 74,306 Int. Cl. B01j 17/18 US. Cl. 23-301 SP 12 Claims ABSTRACT OF THE DISCLOSURE This invention relates to apparatus for growing elongate monocrystalline plates from a melt.

Various methods have been developed for growing monocrystalline bodies from a melt. The present invention concerns apparatus for use in growing crystalline bodies according to what is called the edge-defined, film fed, growth technique (also known as EFG process.) Details of this process are described in Argentine Pat. No. 165,996 dated Apr. 7, 1969 and in the corresponding copending US. patent application of Harold A. La Belle, Jr. Ser. No. 700,126, filed Jan. 24, 1968 for Method of Growing Crystalline Materials.

In the EFG process the shape of the crystalline body is determined by the external or edge configuration of the end surface of a forming member which for want of a better name is called a die. An advantage of the process is that complex shapes can be produced commencing with the simplest of seed crystal geometries, namely, a round small diameter seed crystal. The process involves growth on a seed from a liquid film of feed material sandwiched between the growing body and the end surface of the die, with the liquid in the film being continuously replenished from a suitable melt reservoir by action of capillary rise in a capillary in the die member. Surface tension at the periphery of the film causes it to spread across the full expanse of the end surface of the die until it reaches the edge thereof formed by intersection with another surface of the die. The angle of intersection of the aforesaid surfaces of the die is such relative to the contact angle of the liquid film that the liquids surface tension will prevent it from overrunning the edge of the dies end surface. Preferably the angle of intersection is a right angle which is simplest to achieve and thus most practical to have. The growing body grows to the shape of the film which conforms to the edge configuration of the dies end surface. Since the liquid film has no way of discriminating between an outside edge and an inside edge of the dies end surface, a continuous hole may be grown in the crystalline body by providing in that surface an appropriate hole of the same shape as the hole desired in the growing body, provided, however, that any such hole in the dies end surface is made large enough so that surface tension will not cause the film around the hole to fill in over the hole. From the foregoing brief description it is believed clear that the term edge-defined, film-fed growth denotes the essential feature of the process the shape of the growing crystalline body is defined by the edge configuration of the die and growth takes place from a film of liquid which is constantly replenished.

The primary object of the present invention is to provide an improvement in apparatus and method for growing relatively large size monocrystalline plates of materials such as alumina, by the EFG process. Heretofore it has been found that growth of rectangular plates of sapphire (or-alumina) by the EFG process has been hampered in several ways. For one thing, in growing relatively large size plates, e.g. 3 inches wide by inch thick and 4-6 inches long, it is difficult to control the temperature of the film and the growing body so as to avoid large temperature gradients in a direction transverse to the pulling axis, with the result that the plate produced is characterized by a number of sub-grains and facets. Another problem has been a tendency of the plate to crack on cooling due to internal strain. Accordingly, a specific object of this invention is to grow rectangular plates that have a minimum number of sub-grains, are free of internal strains, and do not crack on cooling. Described briefly the invention consists of improved means for providing a symmetrical temperature distribution about the melt, the growth zone, and the growing body with provision for two-stage heating.

Other features and many of the attendant advantages of this invention are set forth or rendered obvious in the following detailed description which is to be considered together with the accompanying drawing wherein:

FIG. 1 is a fragmentary elevational view, partly in section, of a furnace embodying the present invention;

FIG. 2 is an enlarged view of a portion of the apparatus of FIG. 1;

FIG. 3 is a perspective view of a die assembly for growing rectangular plates that forms part of the apparatus shown in FIG. 2;

FIG. 4 is a cross-sectional view taken substantially along line 4-4 of FIG. 2; and

FIG. 5 is a cross-sectional view of a radiation shield assembly used in the apparatus of FIG. 2.

The present invention may be used to produce rectangular plates of various congruently melting materials that solidify in crystalline form. One such material is ualumina and the following description describes apparatus for growing sapphire plates, i.e. monocrystalline plates of walumina.

Referring now to FIG. 1, there is shown a furnace adapted for use in growing monocrystalline rectangular plates from the melt. The furnace consists of a vertically moveable horizontal bed 2 which engages a stationary furnace enclosure comprising two concentric spaced tubes 4 and 6 which have a rectangular shape in cross-section. Tube 6 is made of quartz and tube 4 is made of quartz or pyrex glass. Attached to the bottom end of the outer tube 6 is a rectangular metal spacer ring 8. The latter has a shoulder 10 as shown which engages the bottom end surface of tube 6 and its inner edge is secured to the outer surface of tube 4. The ring 8 is secured to both tubes by means of a suitable cement such as Silastic which also forms a water tight seal. The ring 8 is sized so as to seat snugly in a gasket 12 positioned in bed 2. Ring 8 is provided with an inlet port fitted with a flexible pipe 16. The upper end of tube 4 is affixed to a head 18 and support means (not shown) are provided for holding the upper end of tube 6 so that it can not move relative to tube 4. Such support means may connect tube 6 to tube 4 or head 18. The upper end of tube 6 terminates in a trough 20 which is provided with an outlet port fitted with a flexible pipe 22. Pipes 16 and 22 are connected to a pump (not shown) that continuously circulates cooling water through the space between the two quartz tubes. The circulating water maintains the inner quartz tube at a safe temperature and absorbs infrared energy so as to make visual observation of crystal growth more comfortable to the observer. The interior of the furnace enclosures is connected by a pipe 26 to a vacuum pump or to a regulated source of inert gas such as argon or helium. The furnace enclosure also is surrounded by an RF. heating coil identified generally at 30 that is coupled to a controllable R.F. power supply (not shown) of conventional construction. The coil 30 has rectangularly-shaped turns sized so as to be evenly spaced from each of the four sides of tube 6. Means (not shown) are provided for supporting the coil in a selected position along the length of the furnace enclosure.

The head 18 is adapted to provide entry into the furnace enclosure of an elongate pulling rod 32 that is connected to and forms part of a conventional crystal pulling mechanism represented schematically at 34. It is to be noted that the type of crystal pulling mechanism is not critical to the invention and that the construction thereof may be varied substantially. Preferably the crystal pulling mechanism is hydraulically controlled since it offers the advantage of being vibration free and providing a uniform pulling speed. Hence, regardless of the exact construction of the pulling mechanism it is to be understood that that mechanism is adapted to move the pulling rod 32 axially at a controlled rate. The pulling rod is disposed coaxially with the quartz tubes 4 and 6 and its lower end has an extension in the form of a metal rod 36 that is adapted to function as a holder for a seed crystal 38.

Located within the furnace enclosure is an assembly constructed according to the present invention. Essentially this assembly comprises a heat susceptor 40 made of carbon. The susceptor 40 is supported on a pair of tungsten support rods 42 that are mounted in bed 2. The susceptor 40 supports a second heat susceptor 44 which also is made of carbon. As explained hereinafter the two susceptors 40 and 44 surround the melt and the growth zone and cooperate to provide a symmetrical temperature distribution about the melt, the growth zone, and the growing crystalline body. Optionally as seen in FIG. 1, the susceptors are surrounded by a radiation shield 50 made of carbon cloth and held in place by carbon yarn (not shown) that is wrapped around it and tied. The purpose of the carbon cloth is to greatly reduce the heat loss from the two carbon susceptors 40 and 44 so as to reduce the amount of RP. power required to reach a given temperature level. Thus, for example, it has been found that the radiation shield 50 will increase the susceptor temperature by as much as 500" C. for a given power setting for the heating coil 30.

Referring now to FIGS. 2 and 4, the susceptor 40 is formed in the shape of a rectangular box open at the top. The upper end of the side walls of susceptor 40 are undercut to provide a continuous peripheral groove 52. The susceptor 44 also is formed in the shape of a rectangular box and the bottom ends of its side walls are undercut to form a rib or tongue extension 54 whereby it will fit together with the susceptor 40 in a tongue and groove connection. The susceptor 44 has a hole 55 at the top for introduction of a seed and also is slotted as shown at 56 so as to provide at its opposite long sides elongate narrow apertures 58 that function as viewing ports whereby the operator may observe growth of a rectangular plate from the melt. The carbon cloth shield has a rectangular configuration in cross section so that it lies rflat against the side walls of the susceptors and, although not shown, it is to be understood that the shield also is cut away to form elongate slots that are aligned with aperture 58 so that the operator may look at the growth zone.

Referring again to FIG. 2, the support rods 42 extend through the susceptor 40 and supported on their upper ends is a rectangular crucible 60 made of tungsten. The height of the crucible is such that the upper end thereof is at or slightly below the level of the viewing apertures 58. The crucible may be made large enough to contain sufiicient feed material to enable growth of a plate of desired size. As an alternative measure, means may be provided to add more feed material to the crucible during plate growth. Such means may be a filler pipe 62 which extends down into the crucible through an opening in susceptor 44. Although not shown, it is to be understood that above the susceptor 42, the filler pipe is angulated and extends out through suitable fittings in the quartz tubes 4 and 6 to a point exterior of the furnace enclosure whereit is accessible to the operator for introduction of feed material. The feed material is preferably introduced in powdered or granular form so that it will pass readily down through the filler pipe into the bottom end of the crucible.

Mounted within the crucible 60 is a rectangular die assembly identified generally by the numeral 64 which is made of tungsten. As seen in FIGS. 2, 3, and 4, the rectangular die assembly 64 consists of a pair of fiat rectangular plates 66 and 68 which are spaced from one another by a pair of spacer blocks 70 and 72 so as to form a passageway 73 which is rectangular in cross-section. The two plates 66 and 68 and the spacer blocks 70 and 72 are secured together by a pair of pins 74 and 76. The bottom end of each of the plates 66 and 68 is formed with two circularly curved, vertically extending slots 78 and 80 which are sized so that the corresponding slots of the two plates form short entry holes for a pair of pins 82 and 84 that are mounted in the bottom of the crucible. The pins 82 and 84 serve to hold the die assembly fixed within the crucible 60'.

In order for the die assembly to satisfy the requirements of the EFG process, the spacing between the two plates 66 and 68 must be such that the passageway 73 will function as a capillary for the melt. Additionally, the relationship between the height of capillary passageway 73 and the aforesaid spacing must be such that the melt in the crucible can rise in the passageway by capillary action to the level of the upper surfaces 86 and 88 of the two plates. For a given melt material the spacing required between plates 66 and 68 to give a predetermined amount of capillary rise or the amount of capillary rise that can be achieved with a predetermined spacing between the two plates can be approximated by the equation where h is the distance in centimeters that the liquid will rise, T is the surface tension of the melt in dynes/cm., D is the density of the melt material in gms./cc., g is the gravitational constant in cm./sec. w is the width of the capillary in centimeters (i.e., the distance between spacer blocks 70 and 72), and t is the depth of the capillary in centimeters (i.e., the distance between plates 66 and 68). For alumina, T is 690 dynes/cm. and D is 3 grams/cm. at the melting point. In this connection it is to be noted that relatively long columns of alumina can be achieved by capillary action. By way of example, if an elongate capillary of circular cross-section has a diameter of 0.75 mm., a column of molten alumina may be expected to rise more than 11 cm. therein. Therefore, it is believed to be apparent that (depending upon the spacing between plates 66 and 68) the overall length of capillary 73 may be of substantial magnitude and still achieve the required amount of capillary rise. As seen in FIG. 2, the die assembly 64 preferably rises to above the level of the crucible 60, although it may terminate flush with or below the upper end of the crucible. The important thing is that the upper surfaces 86 and 88 of the die assembly be above the level of the melt in the crucible at all times and that the differential in height between the level of the melt and said surfaces never exceed the height to which the melt can rise by capillary action. In practice it is preferred that the difference in height be tween the surfaces 86 and 88 and the bottom of the crucible be less than the maximum height that the melt can rise in capillary 73, so that crystal growth can be continued until the melt in the crucible is almost completely consumed.

It is to be noted that the spacer plates 70 and 72 are shown flush with the upper end surfaces 86 and 88 of the plates 66 and 68. However, this is not necessary and the spacer plates may actually terminate below the level of surfaces 82 and 84. In fact, it is also contemplated that the die assembly may be formed without the use of spacer plates 70 and 72. Instead one of the two plates may be formed with a vertical groove therein, such as would result if the spacer plates 70 and 72 were integral with plate 86 or plate 88 and the two plates locked together by rivets or other means, whereby the groove would provide a capillary passageway between the two plates for feeding of melt to the upper surfaces of the die assembly. A further possible modification is to secure the two plates 66 and 68 together solely at their bottom ends so that for a major portion of its length including its upper end, the capillary is open along the vertically extending side edges of the two plates. In such case the spacing required between the two plates to achieve the desired capillary rise or the length of the capillary may be approximated according to the equation -Dg(wt) where h, T, D, g, w, and t are as previously defined. However, in such a case the spacing t between the two plates must be relatively small. It is to be noted also that the top end surfaces 86 and 88 are co-planar and that they intersect the side surface of plates 66 and 68 at right angles.

The assembly of FIG. 2 is completed with a horizontal radiation shield assembly illustrated schematically at 90 made of tungsten. FIG. 5 illustrates a preferred form of horizontal shield assembly. This preferred design comprises a plurality of like size rectangular plates, with the bottom plate 92 being flat and resting on the upper end of the crucible 60, and the other plates 93 and 94 being dimpled as shown at 96 so as to provide spacing between the several plates. Each of the plates 92-94 has a rectangular aperture 98 sized so that the radiation shield as sembly will accommodate and extend close to the die assembly 64.

As seen in FIG. 4, the susceptor 40, crucible 60, and die assembly 64 are symmetrically disposed so that the long sides of the susceptor and crucible extend parallel to the long sides of the plates making up the die assembly. If the filler pipe 62 is used, it is positioned so as to extend down into one corner of the crucible in order that delivery of additional feed material will not unduly disturb the melt in the crucible.

An additional feature of the apparatus of FIG. 2 resides in the disposition of the rectangular heating coil 30. The latter is arranged so as to provide two stage heating, one for the crucible 60 and the susceptor 40 and the other for the susceptor 42. This is achieved by closely spacing a number of its turns surrounding susceptor 40 as shown at 100, while the turns surrounding susceptor 44 are relatively widely spaced as shown at 102. With this arrangement, for a given RF. power input to the coil, the rate of heat input to the susceptor 40 and crucible 60 will be greater than the rate of heat input to the susceptor 44, with the result that the temperature of the melt in the crucible and in the growth zone just above the die assembly will be substantially greater than the temperature of the crystal body 104. The crystal body 104 is kept at an average temperature below its melting point which will relieve strains, i.e. anneal it, and thereby protect it from cracking on cooling. Obviously, the same result can be achieved by using two separate heating coils, one disposed around susceptor 40 and the other disposed around susceptor 44, 'With both coils connected to a suitable power supply through separate controllers so that they may be operated to provide different rates of heat input in the manner described above with respect to coil 30. In this connection it is to be noted that the two susceptors function as heaters or heat radiators, with susceptor 40 serving to supply heat to the crucible and the die, and susceptor 44 serving to supply heat to the crystal body pulled from the melt film on the die.

It is essential that the crucible and the die assembly be made of a metal that will Withstand the operating temperature and will not react with the melt. Thus in growing sapphire plates the crucible, the die assembly, and the radiation shield are made of molybdenum or tungsten as noted above. However, the support rods cannot be made of molybdenum because they contact the carbon susceptor 40 and because molybdenum and carbon have a eutectic and will react at the operating temperature required to grow sapphire plates. If tungsten is used, the crucible is made by welding together flat sheets by electron beam or T.I.G. (tungsten in gas) techniques.

As indicated above the heat susceptor 44 functions as an afterheater for the crystal body that is grown. Without it the crystal body tends to dissipate heat rapidly and will have a sharp temperature gradient in a lateral direction and also in a direction parallel to the pulling axis, notwithstanding the heat input from the turns 102 of coil 30. Depending on the rate of heat loss by radiation this temperature gradient may be great enough to cause small cracks to appear in the crystal body due to thermal shock. More importantly, the high rate of heat loss from the crystal body makes it difi'icult to control the temperature of the film on the upper surface of the die assembly with the result that the power input to the coil 30 has to be adjusted as the crystal body increases in size. These problems are obviated or at least minimized by the presence of susceptor 44. Since its sides extend parallel to corresponding sides of the crystal body, the susceptor 44 functions to minimize temperature gradients and keep the crystal body at a selected temperature suitable to achieve annealing. -As indicated above, the temperature of the crystal body can be adjusted by varying the spacing or the number of the turns of coil 30 surrounding susceptor 44 and the power input to the coil. The lower susceptor 40 functions in a similar manner to minimize temperature gradients in the melt along the plane of the liquid film from which the body is grown. A temperature gradient across the same film, i.e. vertically in FIG. 1, results naturally due to heat loss from the top surface of the film by absorption by the crystal body which functions as a heat sink and also because of the difference in the rate of heat input by the closely spaced heating coil turns as compared to the rate of heat input by more widely spaced turns 102. The thickness of the film is dependent upon its temperature and its rate of heat loss to the crystal body, and as a general statement, its thickness can be increased by increasing its temperature or the pulling speed and can be lowered by decreasing the temperature or pulling speed.

Following is an example of growing rectangular sapphire plates using the above-described apparatus. A tungsten crucible 60 with a molybdenum die member 64 disposed therein is filled to a suitable level with powdered alumina and an -alumina seed crystal is mounted in holder 36. The die assembly has an overall height of about 5.0 cm. and projects about 0.2 above the crucible. The upper end surfaces of the two plates of the die assembly measure 7.5 cm. wide and 0.8 cm. thick. The spaces 70 an 72 are flush with surfaces 86 and 88 and provide a gap between the plates of 0.05 cm. The seed crystal is in the form of a round rod with a diameter of 0.3 cm. so as to melt intothe capillary region. The crucible is then mounted within susceptor 40 on support rods 42. It is to be noted that access to the interior of the furnace enclosure is achieved by lowering bed 2 away from quartz tubes 4 and 6. The radiation assembly 90 (made of molybdenum) is then positioned on the crucible. Then the susceptor 44 is mounted on susceptor 40 as shown. The bed is then raised so as to seal off the furnace enclosure which is then filled with argon via pipe 28 to a pressure of about 1 atmosphere. Coil 30 is then energized from a 500 kc. R.F. supply. The power input to coil 30 is adjusted so that the heat input is sufficient to melt the alumina in the crucible and hold the tempera-ture of the upper end surfaces of the capillary member to a temperature slightly above the melting point of alumina, i.e. above about 2072 C. The coil turns 102 above the level of the upper surfaces of the die assembly are spaced so as to heat the susceptor 44 to a temperature preferably to :within the range of 1850 to 1950 C.

Initially the capillary fills with molten alumina to a height even with the level of the melt in the crucible and then the alumina rises by capillary action to the top of the capillary. Once the capillary is filled, the seed is lowered through hole 55 into contact with the upper surfaces of the die assembly and its end is melted as a result of such contact to form a small area film of melt. After about one minute the seed is raised slowly (at a rate of about 6 inches/hour). If the seed pulls away from the melt film on the upper end of the die assembly, the power input is adjusted to lowerthe temperature of the upper end surface of the die member as required and then the seed is reintroduced into contact with the melt on the upper end surface of the die assembly. The power setting required to achieve a film temperature at which crystal growth will occur is established by trial and error. Once the correct temperature is achieved, growth will appear on the end of the seed, i.e., the seed will not come free of the melt film. Initially the crystal growth will be to about the same diameter as the seed crystal as shown at 106 but then it is made to increase laterally by decreasing the film thickness. This is achieved by decreasing the pulling speed and/or the temperature of the film; however, in no event is the temperature of the upper end of the die assembly lowered to below 2072 C. Once the film has spread to the full area of the upper end of the die member, the height of the growth interface above the die surface (i.e. the film thickness) is maintained constant by controlling the average temperature in this region. The average temperature at the growth interface is monitored by a recording pyrometer which is used to control the RF. power setting. Considerable increases in RF. power input are required to maintain a constant film thickness as the crystal body increases in size due to the increasing heat sink capacity of the latter. The crystal body will grow in cross-sectional area as the film expands and when the film has spread out to cover the full expanse of end surfaces 86 and 88, the growing crystal body 104 will have a rectangular crosssection with one dimension approximately the same as the width of plates 66 and 68 and the other dimension approximately the same as the combined thickness of plates 66 and 68 and the spacer 70 (or 72). The film is constantly replenished with melt via capillary 73. The crystal body 104 will continue to grow to the full size of the end of the die until the melt in the crucible as been consumed to the extent that the film is no longer replenished as above described. However, pulling is continued after crystal growth has terminated until the trailing end of the crystal body has been exposed to the annealing temperature for at least four hours. Then the power is lowered slowly to produce a temperature drop of about 50 C./ hour to about 1400 C., and then the power is dropped faster to produce a temperature drop of about 100 C./ hour to 900 C., and then shut olf and the furnace allowed to cool to a point where the crucible and crystal body can be removed.

Preferably, once the crystal body has expanded in crosssection to the full area of the die member, the pulling speed is adjusted to about 1 inch/hour so as to optimize optical clarity of the product.

Operating according to the procedure just described, it is possible With the apparatus of the present invention to producee sapphire plates that are substantially monocrystalline and do not crack when cooled. The same apparatus has also been used to grow plates of other materials such as spine], and barium titanate, with appropriate changes made in materials of construction and operating conditions according to the reactivity and melting points of the materials to be grown.

It is to be understood that the end surfaces 86 and 88 of the die effectively constitute a single rectangular film supporting surface with a hole therein formed by ca illary 73, and that the capillary may extend to the die's full width (the horizontal dimension as seen in FIG. 4) or be narrower in the form of a rectangular groove or even be a round hole sized to function as a capillary. The die also may be formed with one or more capillaries communicating with its top end surface.

It is to be noted also that the high degree of crystal perfection in the crystal plate produced as above described cannot be achieved if the susceptors 40 and 42 (or the coils of heater 30) are made cylindrical since then certain portions thereof would be spaced further from the melt film and the crystal body than other portions thereof, with the result that substantial thermal gradients transverse to the pulling axis would be present in the melt film and the growing body and such gradients would produce uneven growth across the growth interface between the film and the growing crystal plate.

What is claimed is:

1. Apparatus for use in a furnace for growing a monocrystalline plate of a selected material that crystallizes on solidification onto a seed from a liquid film of said material having a rectangular edge configuration, said apparatus comprising a crucible that is open at the top and is adapted to contain a melt of said material without reacting chemically therewith, a die disposed in said crucible, said die having a top end surface with a generally rectangular edge configuration for supporting said film and a capillary leading from said top end surface to a point adjacent to the bottom of said crucible for feeding said melt from said crucible to said top end surface, and heat susceptor means surrounding said crucible and extending above said crucible a distance sufiicient to surround at least a portion of a monocrystalline plate grown from said film, said heat susceptor means having a vertically extending inside surface of rectangular configuration that is spaced from and is symmetrical with the edge configuration of said top end surface of said die.

2. Apparatus according to claim 1 wherein said susceptor comprises a lower section and an upper section that are separably joined at approximately the level of the top end surface of said die.

3. Apparatus according to claim 1 wherein said heat susceptor has an end wall at its top end, said end wall having an opening for withdrawal of said seed.

4. Apparatus according to claim 1 further including an electrical heater coil surrounding said susceptor, said coil being adapted to supply heat at a first input rate to that portion of said susceptor located below the level of said end surface and at a second lower input rate to that portion of said susceptor located above the level of said end surface.

5. Apparatus according to claim 1 wherein said susceptor has at least one viewing aperture disposed at approximately the level of said top end surface so that an observer may view the solid-liquid interface of a solid growing from a film of melt on said top end surface.

'6. Apparatus according to claim 2 wherein said upper section is supported by said lower section, and further wherein said upper and lower sections cooperate to provide a viewing aperture at approximately the level of the top end surface of said die.

7. Crystal growing apparatus adapted to grow a substantially monocrystalline body of a selected material onto a seed from a film of a melt of said material by the EFG process so that in cross-section said body has a relatively short first dimension transverse to the axis of growth and a relatively long second dimension transverse to the axis of growth and at a right angle to said first dimension, said apparatus comprising a crucible for containing a supply of said melt, a die mounted in said crucible having a rectangular top end surface for supporting said film and a capillary for feeding melt to said end surface from said crucible by action of capillary rise, said top end surface terminating in a sharp edge and hollow susceptor means surrounding and spaced from said crucible and also extending above said crucible so as to radiate heat to said crucible, said die, the film on said die and the crystal body being pulled from said film, said susceptor means comprising a lower section and an upper section joined at approximately the level of the said top end surface of said die, said susceptor means having an inside surface whose configuration in cross-section is similar to the edge configuration of said top end surface.

8. Crystal growing apparatus according to claim 7 including heating means for supplying heat to said susceptor, said heating means comprising an electrical heating coil surrounding said susceptor, the spacing between the turns of said coil being greater above said crucible than at the level of said crucible.

9. In the method of growing a rectangular monocrystalline plate of a selected crystalline material from a liquid film thereof that has a rectangular edge configuration and covers a die surface of corresponding edge configuration and which is continually replenished by feeding additional liquid from a reservoir by capillary action, the improve-,

10 to minimize heat gradients transversely of the direction of growth.

10. A method according to claim 9 further including supplying heat to said film and plate at different rates so that the liquid in said film is maintained at a temperature above the melting point of said material and said growing plate is maintained at a temperature below said melting point sutficient to relieve it of internal stresses.

11. A method according to claim 9 wherein said material is alumina and said plate is maintained at a temperature in the range of 1850 to 1950 C.

12. A method according to claim 9 wherein successive grown portions of said plate are maintained at 1850 to 1950 C. for a period of at least 4 hours.

References Cited UNITED STATES PATENTS 2,975,036 3/1961 Taylor et a1. 23-30l SP 3,031,275 3/1962 Shockley 23-301 SP 3,033,660 5/1962 Okkerse 23-301 SP 3,194,691 7/1965 Dikhoif 23-301 SP 3,212,858 10/1965 Smith 23301 SP 3,342,559 9/1967 Dermatis 23301 SP 3,591,348 7/1971 La Belle 23-301 SP NORMAN YUDKOFF, Primary Examiner S. SILVERBERG, Assistant Examiner US. 01. X.R. 23-473 SP 

